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Muscular System:Histology and Physiology

Chapter 9

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Functions of the Muscular System• Body movement (skeletal muscles attached to bones)

• Maintenance of posture• Respiration (skeletal muscles of thorax are responsible

for the movement necessary for respiration)

• Production of body heat (when skeletal muscles contact, heat is given off as a by-product)

• Communication (speaking, writing)

• Constriction of organs and vessels (contraction of smooth muscle)

• Heart beat (contraction of cardiac muscle)

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General Functional Characteristics of Muscle

• Contractility: ability of a muscle to shorten with force

• Excitability: capacity of muscle to respond to a stimulus (by nerve or hormone)

• Extensibility: muscle can be stretched to its normal resting length and beyond to a limited degree

• Elasticity: ability of muscle to recoil to original resting length after stretched

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Types of Muscle Tissue• Skeletal

– Responsible for locomotion, facial expressions, posture, respiratory movements, other types of body movement

– Voluntary

• Smooth– Walls of hollow organs, blood vessels, eye, glands, skin

– Some functions: propel urine, mix food in digestive tract, dilating/constricting pupils, regulating blood flow

– In some locations, autorhythmic

– Controlled involuntarily by endocrine and autonomic nervous systems

• Cardiac– Heart: major source of movement of blood

– Autorhythmic

– Controlled involuntarily by endocrine and autonomic nervous systems

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Skeletal Muscle Structure• Composed of muscle cells

(fibers), connective tissue, blood vessels, nerves

• Fibers are long, cylindrical, multinucleated

• Tend to be smaller diameter in small muscles and larger in large muscles. 1 mm- 4 cm in length

• Develop from myoblasts (they are converted to muscle fibers as contractile proteins accumulate within their

cytoplasm); numbers remain constant (# of muscle fibers remain constant after birth----so, enlargement of muscles is an increase in size rather than #)

• Striated appearance due to light and dark banding

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Connective Tissue Coverings of Muscle• Layers

– External lamina. Delicate, reticular fibers. Surrounds sarcolemma (P.M.)

– Endomysium. Loose C.T. with reticular fibers.

– Perimysium. Denser C.T. surrounding a group of muscle fibers. Each group called a fasciculus

– Epimysium. C.T. that surrounds a whole muscle (many fascicles)

• Fascia: connective tissue sheet– Forms layer under the skin– Holds muscles together and separates

them into functional groups.– Allows free movements of muscles.– Carries nerves (motor neurons, sensory

neurons), blood vessels, and lymphatics.

– Continuous with connective tissue of tendons and periosteum.

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Nerves and Blood Vessel Supply

• Motor neurons: stimulate muscle fibers to contract. Nerve cells with cell bodies in brain or spinal cord; axons extend to skeletal muscle fibers through nerves

• Axons branch so that each muscle fiber is innervated

• Capillary beds surround muscle fibers

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Muscle Fibers• Nuclei just inside sarcolemma

• Cell packed with myofibrils within cytoplasm (sarcoplasm = cytoplasm without myofibrils)– Threadlike (extends from one end of muscle fiber to the other)

– Composed of protein threads called myofilaments: thin (actin 8nm) and thick (myosin 12nm)

– Sarcomeres: actin & myosin myofilaments form highly ordered units called sarcomeres. They are joined end to end to form the myofibrils.

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Parts of a Muscle

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Actin and Myosin Myofilaments

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Actin (Thin) Myofilaments• Two strands of fibrous (F) actin form a

double helix extending the length of the myofilament; attached at either end at sarcomere.– Composed of G actin monomers

each of which has an active site – Actin site can bind myosin during

muscle contraction. • Tropomyosin: an elongated protein

winds along the groove of the F actin double helix.

•Troponin is composed of three subunits: one that binds to actin, a second that binds to tropomyosin, and a third that binds to calcium ions. Spaced between the ends of the tropomyosin molecules in the groove between the F actin strands. •The tropomyosin/troponin complex regulates the interaction between active sites on G actin and myosin.

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Myosin (Thick) Myofilament

• Many elongated myosin molecules shaped like golf clubs.

• Molecule consists of two heavy myosin molecules wound together to form a rod portion lying parallel to the myosin myofilament and two heads that extend laterally.

• Myosin heads1. Can bind to active sites on the actin molecules to form cross-bridges. 2. Attached to the rod portion by a hinge region that can bend and straighten

during contraction. 3. Have ATPase activity: activity that breaks down adenosine triphosphate

(ATP), releasing energy. Part of the energy is used to bend the hinge region of the myosin molecule during contraction

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Sarcomeres: Z Disk to Z Disk• Z disk: filamentous network of

protein. Serves as attachment for actin myofilaments

• Striated appearance– I bands: from Z disks to ends of

thick filaments– A bands: length of thick

filaments– H zone: region in A band where

actin and myosin do not overlap– M line: middle of H zone;

delicate filaments holding myosin in place

•In muscle fibers, A and I bands of parallel myofibrils are aligned.•Titin filaments: elastic chains of amino acids; make muscles extensible and elastic

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Sliding Filament Model• Actin myofilaments sliding over

myosin to shorten sarcomeres– Actin and myosin do not change

length– Shortening sarcomeres

responsible for skeletal muscle contraction

• During relaxation, sarcomeres lengthen because of some external force, like forces produced by other muscles (contraction of antagonistic muscles) or by gravity.

- agonist = muscle that accomplishes a certain movement, such as flexion.

- antagonist = muscle acting in opposition to agonist.

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Sarcomere Shortening

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Physiology of Skeletal Muscle Fibers

• Nervous system controls muscle contractions through action potentials• Resting membrane potentials

– Membrane voltage difference across membranes (polarized)• Inside cell more negative due to accumulation of large protein molecules.

More K+ on inside than outside. K+ leaks out (through leak channels) but not completely because negative molecules hold some back.

• Outside cell more positive and more Na+ on outside than inside. • Na+ /K+ pump maintains this situation.

– Must exist for action potential to occur

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Ion Channels• Types

– Ligand-gated. Ligands are molecules that bind to receptors. Receptor: protein or glycoprotein with a receptor site

• Example: neurotransmitters• Gate is closed until neurotransmitter attaches to

receptor molecule. When Ach (acetylcholine) attaches to receptor on muscle cell, Na gate opens. Na moves into cell due to concentration gradient

– Voltage-gated• Open and close in response to small voltage changes

across plasma membrane

• Each is specific for one type of ion

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Action Potentials• Phases

– Depolarization: Inside of plasma membrane becomes less negative. If change reaches threshold, depolarization occurs

– Repolarization: return of resting membrane potential. Note that during repolarization, the membrane potential drops lower than its original resting potential, then rebounds. This is because Na plus K together are higher, but then Na/K pump restores the resting potential

• All-or-none principle: like camera flash system• Propagate: Spread from one location to another.

Action potential does not move along the membrane: new action potential at each successive location.

• Frequency: number of action potential produced per unit of time

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Gated Ion Channels and the Action Potential

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Action Potential Propagation

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Neuromuscular Junction

• Synapse: axon terminal resting in an invagination of the sarcolemma

• Neuromuscular junction (NMJ):– Presynaptic terminal:

axon terminal with synaptic vesicles

– Synaptic cleft: space– Postsynaptic membrane

or motor end-plate

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Function of Neuromuscular Junction

• Synaptic vesicles– Neurotransmitter: substance

released from a presynaptic membrane that diffuses across the synaptic cleft and stimulates (or inhibits) the production of an action potential in the postsynaptic membrane.

• Acetylcholine – Acetylcholinesterase: A

degrading enzyme in synaptic cleft. Prevents accumulation of ACh

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Excitation-Contraction Coupling

• Mechanism by which an action potential causes muscle fiber contraction

• Involves– Sarcolemma

– Transverse (T) tubules: invaginations of sarcolemma

– Terminal cisternae

– Sarcoplasmic reticulum: smooth ER

– Triad: T tubule, two adjacent terminal cisternae

– Ca2+

– Troponin

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Action Potentials and Muscle Contraction

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Cross-Bridge Movement

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Relaxation

• Ca2+ moves back into sarcoplasmic reticulum by active transport. Requires energy

• Ca2+ moves away from troponin-tropomyosin complex

• Complex re-establishes its position and blocks binding sites.

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Muscle Twitch• Muscle contraction in

response to a stimulus that causes action potential in one or more muscle fibers

• Muscle contraction measures as force, also called tension. Requires up to a second to occur.

• Phases– Lag or latent

(neuromuscular junction & step #1 of cross-bridge movement)

– Contraction (step #2 - #6 of cross-bridge movement)

– Relaxation (powerpoint slide # 28)

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Stimulus Strength and Muscle Contraction

• All-or-none law for muscle fibers– Contraction of equal force in response to

each action potential

• Sub-threshold stimulus: no action potential; no contraction

• Threshold stimulus: action potential; contraction

• Stronger than threshold; action potential; contraction equal to that with threshold stimulus

• Motor units: a single motor neuron and all muscle fibers innervated by it

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Contraction of the Whole Muscle • Whole muscles exhibit characteristics that are more complex than

those of individual muscle fibers or motor units. Instead of responding in an all-or-none fashion, whole muscles respond to stimuli in a graded fashion, which means that the strength of the contractions can range from weak to strong.

• Remember: There are many muscle fibers in one fasciculi and many fasciculi in one whole muscle.

• Strength of contraction in whole muscle is graded: ranges from weak to strong depending on stimulus strength

• Multiple motor unit summation: the force in which a whole muscle contracts depends on the number of motor units stimulated to contract. (force of contraction increases as more & more motor units are

stimulated). A muscle has many motor units– Submaximal stimuli

– Maximal stimulus

– Supramaximal stimuli

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Contraction of the Whole Muscle

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Stimulus Frequency and Muscle Contraction

• Relaxation of a muscle fiber is not required before a second action potential can stimulate a second contraction.

• As the frequency of action potentials increase, the frequency of contraction increases– Incomplete tetanus: muscle fibers partially relax between contraction– Complete tetanus: no relaxation between contractions– Multiple-wave summation: muscle tension increases as contraction

frequencies increase

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Types of Muscle Contractions

• Isometric: no change in length of muscle but tension increases during contraction– Postural muscles of body ex: muscles hold spine erect while

person is sitting or standing

• Isotonic: change in length but tension constant ex: waving using computer keyboard

– Concentric: tension is so great it overcomes opposing resistance and muscle shortens ex: raising of a weight during a bicep curl.

– Eccentric: tension maintained but muscle lengthens ex: person slowly lowers a heavy weight

• Muscle tone: constant tension by muscles for long periods of time

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Fatigue

• Decreased capacity to work and reduced efficiency of performance

• Types– Psychological: depends on emotional state of

individual ex: burst of activity in tired athlete in response to encouragement from spectators shows how psychological fatigue can be overcome

– Muscular: results from ATP depletion ex: fatigue in lower limbs of marathon runners or in upper & lower limbs of swimmers

– Synaptic: occurs in NMJ due to lack of acetylcholine ex: rare-----only under extreme exertion

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Physiological Contracture and Rigor Mortis

• Physiological contracture: state of extreme fatigue (extreme exercise) where due to lack of ATP neither contraction nor relaxation can occur

• Rigor mortis: development of rigid muscles several hours after death. Ca2+ leaks into sarcoplasm and attaches to myosin heads and crossbridges form but no ATP available to bind to myosin---------so the cross-bridges are unable to release. Rigor ends as tissues start to deteriorate.

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Energy Sources

• ATP provides immediate energy for muscle contractions. Produced from three sources– Creatine phosphate

• During resting conditions stores energy to synthesize ATP• ADP + Creatine phosphate------------------ Creatine + 1ATP (Creatine Kinase)

– Anaerobic respiration• Occurs in absence of oxygen and results in breakdown of

glucose to yield ATP and lactic acid

– Aerobic respiration• Requires oxygen and breaks down glucose to produce ATP,

carbon dioxide and water• More efficient than anaerobic

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Slow and Fast Fibers• Slow-twitch oxidative

– Contract more slowly, smaller in diameter, better blood supply, more mitochondria (also called oxidative because carry out aerobic respiration), more fatigue-resistant than fast-twitch, large amount of myoglobin (dark pigment which binds oxygen & acts as a muscle reservoir for oxygen when blood does not supply adequate amount).

– Postural muscles, more in lower than upper limbs. Dark meat of chicken.– Functions: Maintenance of posture & performance in endurance activities.

• Fast-twitch – Respond rapidly to nervous stimulation, contain myosin that can break down ATP

more rapidly than that in Type I, less blood supply, fewer and smaller mitochondria than slow-twitch (adapted to perform anaerobic respiration)

– Lower limbs in sprinter, upper limbs of most people. White meat in chicken.– Comes in oxidative and glycolytic forms– Functions: Rapid, intense movements of short duration

• Distribution of fast-twitch and slow-twitch– Most muscles have both but varies for each muscle

• Exercise: weight lifting enlarges fast-twitch & aerobic training enlarges slow-twitch• Effects of exercise: change in size of muscle fibers

– Hypertrophy: increase in muscle size• Increase in myofibrils• Increase in nuclei due to fusion of satellite cells• Increase in strength

– Atrophy: decrease in muscle size• Reverse except in severe situations where cells die

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Smooth Muscle• Not striated, fibers smaller than those in skeletal muscle• Spindle-shaped; single, central nucleus• More actin than myosin• Caveolae: indentations in sarcolemma; may act like T tubules• Dense bodies instead of Z disks as in skeletal muscle; have noncontractile

intermediate filaments• Ca2+ required to initiate contractions; binds to calmodulin (protein). Calmodulin

molecules with Ca++ bound to them activate an enzyme called myosin kinase, which transfers a phosphate group from ATP to heads of myosin molecules. Cross-bridging occurs

• Relaxation: caused by enzyme myosin phosphatase

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Electrical Properties of Smooth Muscle

• Slow waves of depolarization and repolarization transferred from cell to cell

• Depolarization caused by spontaneous diffusion of Na+ and Ca2+ into cell

• Does not follow all-or-none law

• Contraction regulated by nervous system and by hormones (ex: epinephrine)

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Regulation of Smooth Muscle

• Innervated by autonomic nervous system (composed of nerve fibers that send impulses from CNS to smooth muscle, cardiac muscle, glands)

• Neurotransmitters are acetylcholine and norepinephrine (increases cardiac output, blood glucose levels)

• Hormones important as epinephrine and oxytocin

• Receptors present on plasma membrane; which neurotransmitters or hormones bind determines response

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Cardiac Muscle

• Found only in heart• Striated• Each cell usually has one nucleus• Has intercalated disks and gap junctions• Autorhythmic cells• Action potentials of longer duration • The depolarization of cardiac muscle results from

influx of Na+ and Ca2+ across the plasma membrane

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Effects of Aging on Skeletal Muscle

• Reduced muscle mass

• Increased time for muscle to contract in response to nervous stimuli

• Reduced stamina

• Increased recovery time

• Loss of muscle fibers